Hello and thank you for looking into your catalogues. And in the meantime I really found it in the APEX database, it's certificate 173358 from 2007.
https://classic.stamps.org/ShowStam...g/173358.jpgSo now we know the source of it. When I look at the image, it could also be a lake color. But as always we can't be sure because of the image processing. In each case, this color is kind of "familiar", I am sure I have seen it before on this design, but perhaps on the 634 or perf 10.
By the way it's great that you can see the certificates / stamps at APEX and search for them, PF also offers this (and Siegel for their auctions, of course), only PSE does not offer a search function, correct? (the cert. ID search is not useful for finding a Scott ID or keyword...)